The present invention relates to a device for limiting the lateral buckling of the tensile armor plies of a flexible pipe.
The flexible pipe at which the present invention is aimed is, in particular, of the unbonded type and used in offshore production. Flexible pipes as defined in the American Petroleum Institute recommendation API 17J generally comprise one or more polymer layers and metal reinforcing layers such as tensile armor plies, a carcass and/or a pressure vault. The nature, number, size and organization of these layers are essentially determined by the conditions of use of the flexible pipes concerned and by the way they are laid, as defined in recommendation API 17J.
When the flexible pipe, regardless of its nature, is subjected to an external pressure Pe that is higher than the internal pressure, axial compression may arise and this is known as the reverse end-wall effect. The reverse end-wall effect has a tendency to compress the armor and to shorten the length of the flexible pipe and increase its diameter, this increase in diameter has the effect of causing the armors to swell. Under certain conditions, for example with an unsealed external sheath and regardless of whether the pipe is straight or curved, the armors can buckle in a radial mode and take on a “bird cage” shape. Another mode of buckling of the tensile armors due to the compressive stresses that they experience due to the reverse end-wall effect is known as lateral buckling, which can arise when the flexible pipe is bent and regardless of the condition of the external sheath. This lateral buckling is often accompanied by overlapping of the armor wires of any ply which, when they part laterally under too high a stress, overlap the armor wire next to them.
When the polymer external sheath of the flexible pipe is punctured for any reason, the pressure in the annulus, which is delimited between said external sheath and the pressure sheath and in which the tensile armors are placed, is equal to the hydrostatic pressure. Under such conditions, the external sheath is no longer pressed firmly against the tensile armors and radial swelling of said tensile armors, likely to arise following the ingress of water into the annulus, is no longer prevented by the external sheath.
Furthermore, the friction existing between the armor plies and generated by the combination of the effects of the external pressure and of the dynamic stresses is likely to cause the wires to migrate and therefore to cause a localized increase in the lateral clearances between the wires of one and the same armor ply. The appearance of these clearances leads, under certain conditions, to lateral buckling of the armor wires, this occurring regardless of the condition of the external sheath.
One of the solutions adopted for reducing the risk of lateral and/or “bird cage” radial buckling and for reducing the armor swelling connected with the reverse end-wall effect was to wind tapes or layers of aramid fibers such as “Kevlar” around the last armor ply. In that way, the armor plies were thus allowed to swell by an amount ΔR which had to be less than half the thickness of the last armor wire. Thus, the swelling of the armor plies was limited while also reducing the risk of the armor plies overlapping.
However, while this solution is able to solve the problems associated with radial buckling, all it can do is limit the risk of lateral buckling, which remains. This is because, depending on the conditions of use of the flexible pipe and, in particular, when this pipe experiences static stresses due to the reverse end-wall effect which are combined with variations in curvature and with dynamic stresses in use, the phenomenon of lateral buckling remains. Now, when an armor wire moves laterally as a result of lateral buckling, it carries with it the other armor wires of the ply. The result is that the flexible pipe is, if not destroyed, at least rendered unservicable, and has to be changed because repairing a flexible pipe is not an economically viable proposition.
In a flexible pipe comprising a sealed external sheath, the rubbing of the plies against each other generates contact pressure which causes the armor wires to move; the experts say that the armor wires “migrate”. As a result of these successive and, repeated movements, the armor wires end up becoming inelastic because of plastic deformation. These migrations of the armor wires may lead to the formation of a small clearance between the successive turns and, during compression or shortening of the flexible pipe, lateral buckling of the armors occurs.
Already known, from the documents U.S. Pat. No. 5,730,188, WO 01/33129 or U.S. Pat. No. 5,275,209, are windings of antiwear layers wound around the tensile armor layers. These antiwear layers are known in API 17J. They are made of a polymer and their sole purpose is to reduce the wear of the plies by reducing friction between the plies, preventing metal-to-metal contact of the shaped wires that make up the plies. These layers have no effect on lateral buckling.